The present invention relates to an optical encoder for detecting position, moving speed, moving direction and the like of a mover with use of light receiving elements. In particular, the invention relates to an optical encoder suitable for use in copiers, printers or other printing devices, FA (Factory Automation) equipment, and the like.
Conventionally, JP H1-136019 A discloses an encoder in which pulse signals having a phase difference of 90° are inputted to an EXCLUSIVE-OR circuit, allowing a resolution twice higher than conventional ones to be obtained.
Also, JP H1-156621 A discloses an optical encoder in which four signals different in phase by 45° are inputted to an EXCLUSIVE-OR circuit to obtain two signals different in phase by 90°.
Also, JP H3-37518 A discloses an encoder in which a signal obtained by EXCLUSIVE-ORing signals of a 90° phase difference is inputted as a clock for a shift register while one of the original signals of a 90° phase difference is used as data input in order to read forward and reverse rotations of a rotor.
Also, JP H7-12924 U discloses an encoder in which 2n-1 rectangular-wave signals are created from 2′ optical sensor outputs so that the resolution is increased twofold.
Also, JP 2007-40959 A discloses an encoder in which a plurality of signals of different phases obtained from light receiving elements, which are subdivided for mover slits, are EXCLUSIVE-ORed to output a signal having a frequency higher than an on/off frequency by the mover.
Generally, for encoders, it is known that a twice higher resolution can be obtained from pulse signals having a 90° phase difference by using an EXCLUSIVE-OR circuit as shown in the above JP H1-136019 A and JP H1-156621 A.
However, in the case where two pulse signals are EXCLUSIVE-ORed as shown above, there inevitably occurs a 1-gate delay within the logic circuit.
In ordinary logic circuits, that delay falls within an error range. However, in a light receiving circuit including light receiving elements that treat weak signals, the 1-gate delay causes impedance differences between signal input and output, which leads to variations in the duty ratio of an output signal, with the periodic fluctuation increased. As a result, there is a problem that merits by twice higher resolutions could not be obtained.
Also, in JP H3-37518 A, logically combining a signal obtained by EXCLUSIVE-ORing with an original signal before the logical operation of EXCLUSIVE-ORing would involve a further gate delay, resulting in a easier occurrence of malfunction.
Also, in JP H7-12924 U and JP 2007-40959 A, although signal pulses are produced by using signal of reverse phase obtained from light receiving elements, yet inputting two pulses having a 90° phase difference to the logical operation input section would cause the gate delay to occur.